1. Field of the Invention
The present invention relates to an audio amplifier. More particularly, the present invention relates to an audio amplifier that can reduce the common mode voltages of the differential mode output signals and reduce the silicon area used.
2. Description of Related Art
FIG. 1 shows a schematic circuit block diagram of a conventional audio amplifier. Referring to FIG. 1, generally, when an audio amplifier 100 is operating, voltages of output signals Voutp and Voutn will be higher than voltages of input signals Vinp and Vinn received by an amplifier 110. Moreover, the audio amplifier 100 outputted signals, Voutp and Voutn, may both be at a logic high voltage level 1. Thus, the output signals Voutp and Voutn have high common mode voltages. Since the output signals Voutp and Voutn will feedback to the input terminal of the amplifier 110, the voltages of the input signals Vinp and Vinn of the amplifier 110 are elevated as well.
Therefore, to the amplifier 110 using a PMOS input pair, as the voltages of the input signals Vinp and Vinn are elevated, a dimension of the PMOS input pair has to be increased. Therefore, the element dimension (W/L) of the PMOS is increased, and too much silicon area will be wasted as a consequence. Moreover, the audio amplifier 100 can not operate normally if the voltages of the input signals Vinp and Vinn of the amplifier are too high.